Metal halide lamps began with the addition to the high pressure mercury lamp of the halides of various light-emitting metals in order to modify its color and raise its operating efficacy as proposed by U.S. Pat. No. 3,234,421--Reiling, issued in 1966. Since then metal halide lamps have become commercially useful for general illumination; their construction and mode of operation are described in IES Lighting Handbook, 5th Edition, 1972, published by the Illuminating Engineering Society, pages 8-34.
The light-emitting metals favored by Reiling for addition to the arc tube fill were sodium, thallium and indium in the form of iodides. This combination had the advantage of giving a lamp starting voltage almost as low as that of a mercury vapor lamp, thus permitting interchangeability of metal halide with mercury lamps in the same sockets. A later U.S. Pat. No. 3,407,327--Koury et al issued in 1968, proposed as additive metals sodium, scandium and thorium; that fill is now favored because it produces light of somewhat better spectral quality. Unfortunately, it also entails a higher starting voltage so that the lamp is not generally interchangeable with mercury vapor lamps.
In the earlier thallium-containing metal halide lamps, the electrodes used comprised tungsten coils carrying thorium oxide in the turns. In operation, the thorium oxide is believed to decompose slightly and release free thorium to supply a monolayer film having reduced work function and higher emission. Unfortunately, this cathode cannot be used in a scandium-containing lamp because the ScI.sub.3 is converted to Sc.sub.2 O.sub.3, resulting in loss of essentially all the scandium in a relatively short time. Instead a thorium-tungsten electrode is used which is formed by operating a tungsten cathode, generally a tungsten rod having a tungsten coil wrapped around it to serve as a heat radiator, in a thorium iodide-containing atmosphere. Under proper conditions the rod acquires a thorium spot on its distal end which serves as a good electron emitter and which is continually renewed by a transport cycle involving the halogen present which returns to the cathode any thorium lost by any process. The thorium-tungsten electrode and its method of operation are described in Electric Discharge Lamps by John F. Waymouth, M.I.T. Press, 1971, Chapter 9.
We find that the proper operation of the thorium transport cycle is suppressed when excess iodine is present. In a cool lamp at room temperature the excess iodine is present as HgI.sub.2. When the lamp operates, this mercury iodide decomposes and the free iodine reacts with the thorium at the electrode. The thorium concentration at the electrode tip is governed by the equilibrium expression: EQU Th(c)+4I(g).revreaction.ThI.sub.4 (g)
In the presence of high iodine concentrations, the forward reaction favoring the formation of ThI.sub.4 predominates. At sufficiently high iodine concentrations, no thorium is deposited on the electrode at all, and the result is a high work function electrode. The electrode must then run hotter to sustain the arc current and this entails lower efficiency most noticeable in the smaller sizes of lamps. The higher temperature makes the lamp blacken due to tungsten evaporation and the result is a poor maintenance lamp.
In one manufacturing process, the lamps are dosed with mercury as liquid and with the iodides of Na, Sc, and Th in pellet form. In this process, it is practically unavoidable that some hydrolysis reaction occurs due to absorption of moisture from the atmosphere by the pellets in transferring them to the lamp envelope. The metal halide dose comprising NaI, ScI.sub.3 and ThI.sub.4 is extremely hygroscopic and even very low levels of moisture will result in some hydrolysis. The hydrolysis results in conversion of the metal halide to oxide with release of HI, for example: EQU 2ScI.sub.3 +3H.sub.2 O.fwdarw.Sc.sub.2 O.sub.3 +6HI
The HI reacts with mercury to form HgI.sub.2 which is relatively unstable at high temperatures, and when the lamp warms up, the HgI.sub.2 decomposes and releases free iodine. Some excess iodine also is frequently found in the dosing materials, possibly as a byproduct of the synthesis of these materials. The result is a lamp which frequently contains excess iodine from the start.
In another manufacturing process, part of the mercury and the halogen component of the charge are introduced into the lamp envelope in the form of HgI.sub.2 and scandium and thorium are added as elements. By varying the ratio of Hg to HgI.sub.2, the iodine may be made substoichiometric relative to the Sc or Th present, in which case the lamp begins its life with no excess iodine. However we have found that a slow reaction between the scandium and thorium iodides and the fused silica arc tube gradually frees iodine during the course of the lamp's life. As the free iodine concentration builds up, a point is reached where thorium ceases to be deposited on the electrode at all and the result is a high work function electrode.
Thus prior art lamps, no matter by what process made and even when they begin life without an excess of iodine, eventually arrive at a condition of excess iodine concentration which reduces lamp efficacy and results in an increased rate of blackening and lumen depreciation. The object of the invention therefore is to provide control of excess iodine throughout the full period of the lamp's life in order that the lamp have higher efficiency, better maintenance and a longer useful life.